This invention relates to a method of producing an array of MIM type devices, together with associated address conductors and pad electrodes, on a common support for use particularly, although not exclusively, in active matrix addressed display devices, for example liquid crystal display devices.
Known MIM devices, generally comprising a thin film insulating layer sandwiched between two conductive layers across which in use a voltage is applied, can be regarded as a kind of diode structure in that they exhibit a non-linear resistive characteristic and have been used in active matrix addressed liquid crystal display devices as switching elements in the addressing of the display device's picture elements. These two terminal devices offer advantages over TFTs also used for such purposes in that they are comparatively simple to fabricate and require fewer address lines, with no cross-overs, on their supporting substrate.
Examples of arrays of MIM devices for liquid crystal display devices and their methods of fabrication are described in U.S. Pat. Nos. 4413883 and 4683183. The display devices consist of first and second glass substrates carrying respectively sets of row and column address conductors with individual picture elements being provided at the region of the intersections of the crossing conductors. A picture element electrode carried on the first substrate is connected electrically to a row conductor via at least one MIM device which is also carried on the first substrate. The MIM devices act as bidirectional switches controlling operation of their associated picture elements. By virtue of their non-linear resistance behaviour, the devices exhibit a threshold characteristic and in operation are turned on in response to a sufficiently high applied field to allow video data signal voltages to be transferred to the picture elements to cause the desired display response.
Although such a device is generally referred to as a Metal-Insulator-Metal device, conductive materials such as indium tin oxide (ITO) can be used as one or both of the "metal" layers and the acronym should be construed accordingly. Moreover, the terms "insulator" and "insulating layer" as used herein are intended to be construed in the wider sense to include semi-insulators and non-stoichiometric materials known in the field of MIM devices. The switching characteristics are dependent on the composition and thickness of the insulating layer and are determined by the charge transfer mechanisms involved. The switching behaviour of many MIM devices results from tunnelling or hopping of carriers in the thin film insulating layer and in this respect the voltage/resistance characteristic of the device is dependent on the magnitude of the electric field and thus the nature and thickness of the insulating layer. In some forms of MIM devices the mechanism is controlled by the barrier between the metal and the (semi-) insulator.
The aforementioned specifications describe various forms of MIM devices using different materials. For the conductive layers, these can include nickel, chromium, tantalum, aluminium or ITO. The insulator layer may be of tantalum pentoxide, silicon nitride, silicon dioxide, silicon oxynitride, silicon monoxide or zinc oxide. Further examples of MIM structures used in display devices, comprising non-stoichiometric materials, are described in EP-A-0182484.
Active matrix addressed types of display devices are comparatively expensive to produce. Although active matrix substrates of display devices using MIM devices are generally simpler to construct, and thus less expensive, than those using TFTs as switching elements, there is still a need for further improvements in the manufacturing processes. Methods of fabricating arrays of MIM type devices with picture element electrodes and address conductors on substrates described in the aforementioned specifications involve a plurality of photolithographic patterning processes using separate masks and separate exposures which, besides requiring expensive equipment, can also lead to problems, especially with the need for accurate alignment and registration. These problems become even more significant when fabricating comparatively large area or high density arrays or arrays on flexible substrates.